In the fields of semiconductor devices, flat-panel displays, MEMSs, and the like, structures with microscopic patterns formed on their surfaces (hereafter, referred to as “microscopic structures”) are manufactured by the lithography technique and the like. In recent years, smaller and more highly integrated microscopic structures have been manufactured. Along with such trends, the patterns formed on the surfaces of such microscopic structures have become finer.
Methods of inspecting such patterns include, for example, an inspection method called as the die-to-die inspection method. In this inspection method, firstly, identical patterns formed at different positions on the surface of an object to be inspected (hereafter, referred to as “workpiece”) are detected by a detector. Then, the pieces of the detected data are compared to each other to find out whether there are or are not any defects or foreign objects (hereafter, simply referred to as “defects”). Unlike the die-to-database inspection method, the die-to-die inspection method does not need to create reference data from the design data (CAD data) in accordance to the pattern. Accordingly, the use of the die-to-die inspection method makes it possible to simplify the pattern inspection apparatus and the pattern inspection method.
As patterns become finer these days, defects formed in manufacturing processes have become more microscopic. Under such circumstances, if the size of a defect becomes smaller relative to the wavelength of the illuminating light, the amount of light scattered by the defect becomes smaller. As a result, a difference in the reflectance due to existence of a defect becomes smaller, so that the contrast is lowered.
To address this problem, a pattern inspection apparatus configured as follows has been proposed (refer to JP-A 8-327557 (Kokai)). The apparatus includes a view-field dividing unit, a shift-adjustment unit, and a defect highlighting unit. The view-field dividing unit divides an acquired optical image into two optical images which are laterally shifted from each other within the plane of the acquired image. The shift-adjustment unit laterally shifts the two optical images to superpose one upon the other. The defect highlighting unit detects a portion with defect by superposing the two optical images and thus optically deleting portions where there are no defects from the pattern.
The technique disclosed in JP-A 8-327557 (Kokai) uses the interference of the reflected lights with each other to delete the optical image corresponding to the portions without any defects. If any of the inspected patterns has a defect, the optical image corresponding to the portion with the defect remains undeleted. Thus, the apparatus of JP-A-8-327557 (Kokai) can check for defects.
According to the technique disclosed in FIG. 2 and the like of JP-A 8-327557 (Kokai), the beams of reflected light from different patterns are made to substantially coaxially enter. The beams of light thus having entered are divided into two optical images. Then the optical images are laterally shifted, and superposed one upon the other. The technique, however, has the following problems. The contrast corresponding to a microscopic defect may not be enhanced when the beams of reflected light entering substantially coaxially from the different patterns are divided with insufficient accuracy.
In addition, as disclosed in FIG. 11 of JP-A 8-327557 (Kokai), the apparatus is capable of detecting defects, but is incapable of increasing the intensities of light corresponding to the portions with defects. For this reason, the apparatus may fail to enhance the contrast corresponding to a microscopic defect.
Microscopic structures may have various defects. For example, each of the defects may differ in: type such as a short-circuited pattern, conduction, depletion, foreign objects remaining in the structure; material such as oxides, nitrides, metals, and semiconductors; and shape such as dimensions of the defects in the longitudinal and lateral directions. This causes a problem of variation in the wavelength and polarization of the light irradiating when the most appropriate contrast is to be obtained. Moreover, since the defects vary in type, material, and shape as described above, there are two types of defects: one which causes decrease in amount of reflected light thereby detected as a negative contrast; and the other which causes increase in amount of reflected light thereby detected as a positive contrast. In addition, since the defects vary in type, material, and shape as described above, beams of light may cancel off each other under certain interference conditions, and, as a result, the contrast may be lowered.